Material and Bond Properties of Frost-Damaged Concrete
Material and bond properties of frost-damaged concrete were studied experimentally. Tests were performed on a reference concrete of grade C35/45 and frost-damaged concrete of two damage levels I and II, corresponding to 25 and 50% reduction in compressive strength caused by freeze-thaw cycles. The level of frost damage was monitored and quantified by ultrasonic measurements, fundamental frequency test and number of freeze-thaw cycles. It was observed that the results from ultrasonic measurements on frost-damaged concrete were more stable and reliable compared to the results from fundamental frequency test; therefore, ultrasonic measurements together with number of freeze-thaw cycles were the basis for frost damage quantification in this study.
The behavior of frost-damaged concrete was evaluated in compression by compression and elastic modulus tests, and in tension by splitting tensile and wedge splitting tests. The effect of frost on bond properties between concrete and reinforcement was assessed using pull-out test. Finally, the resistance of the reference concrete to freeze-thaw attack was evaluated using frost resistance test.
The test results showed significant influence of freeze-thaw cycles not only on the compressive strength but, to a larger extent, on the modulus of elasticity and the compressive strain at peak stress. Thus, the shape of the stress-strain curve of frost-damaged concrete in compression was influenced. Regarding the behavior in tension, reduced tensile strength and increased fracture energy were measured. The latter can be explained as the effect of several micro-cracks introduced to concrete due to freeze-thaw cycles prior to mechanical loading. Therefore, the fracture energy measured included not only opening of one crack, but several micro-cracks. This explains the higher dissipated energy. Pull-out tests also showed significant effect of freeze-thaw cycles on the bond capacity of frost-damaged concrete. However, the slip at maximum bond was not considerably influenced by frost.